Abstract Cortical interneurons represent a broad class of inhibitory neurons that are essential for controlling brain excitability and coordinating behavior. Disruption of inhibitory circuits has been implicated in a number of brain disorders, including epilepsy, intellectual disability, autism, schizophrenia, and head injury. Recently, advances in mouse and human stem cell research suggest that pluripotent cells can be used to generate enriched populations of cortical neurons and interneurons in vitro. However, few studies have systematically examined how exogenous inhibitory neurons derived from stem cell sources might be used as a cell-therapy to modify neural circuitry in vivo. The overall goal of this K99/R00 application is to determine how cortical interneurons derived from mouse and human induced pluripotent stem cells (iPS cells) functionally incorporate into the postnatal brain. The mentored phase of the award will be conducted at University of California, San Francisco under the guidance of Dr. Scott Baraban and the project will be continuted in my own laboratory after an independent faculty position is secured. In Specific Aims 1 and 2, I will use a promoter-based reporter construct to purify coritcal interneuron precursors generated from iPS cells and characterize their differentiation in vitro (Aim 1) and after transplantation (Aim 2) using a series of anatomical, molecular, and electrophysiological approaches. In Aim 3 (R00 phase), I will determine the connectivity patterns of iPS cell- derived interneurons grafted into the postnatal brain. Understanding how cortical interneurons generated from stem cell sources functionally incorporate into the recipient circuitry will provide new information about their functional plasticity and is a critical step toward translating these findings into new interneuron-based cell therapies. My long term goal is to build an independent dedicated to understanding mechanisms of neural circuit organization and to develop novel stem cell strategies for brain repair and regeneration, particularly for brain disorders associated with interneuron dysfunction. This research will require extensive training in stem cell biology, and UCSF is an outstanding institution to complete the mentored phase of this application, primarily due to the rich community of prominent neuroscientists and clinicians performing neural stem cell research and the pioneering role of UCSF in the stem cell field. In addition to Dr. Baraban's outstanding mentorship, I have assembled a team of internationally recognized scientists who will provide me with hands- on technical training, formal coursework, and career guidance during both phases of this proposal. Overall, these training experiences will be critical for me to successfully obtain an academic faculty position and establish my independent research program.